<!DOCTYPE html>



  


<html class="theme-next pisces use-motion" lang="zh-Hans">
<head><meta name="generator" content="Hexo 3.8.0">
  <meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
<meta name="theme-color" content="#222">

<script>
    (function () {
        if ('') {
            if (prompt('请输入文章密码') !== '') {
                alert('密码错误！');
                if (history.length === 1) {
                    location.replace("https://xczhello.gitee.io/"); // 这里替换成你的首页
                } else {
                    history.back();
                }
            }
        }
    })();
</script>








<meta http-equiv="Cache-Control" content="no-transform">
<meta http-equiv="Cache-Control" content="no-siteapp">
















  
  
  <link href="/lib/fancybox/source/jquery.fancybox.css?v=2.1.5" rel="stylesheet" type="text/css">







<link href="/lib/font-awesome/css/font-awesome.min.css?v=4.6.2" rel="stylesheet" type="text/css">

<link href="/css/main.css?v=5.1.4" rel="stylesheet" type="text/css">


  <link rel="apple-touch-icon" sizes="180x180" href="/images/favicon.jpg?v=5.1.4">


  <link rel="icon" type="image/png" sizes="32x32" href="/images/favicon.jpg?v=5.1.4">


  <link rel="icon" type="image/png" sizes="16x16" href="/images/favicon.jpg?v=5.1.4">






  <meta name="keywords" content="Hexo, NexT">










<meta name="description" content="1. 渲染机制浏览器的组成  用户界面 － 包括地址栏、后退/前进按钮、书签目录等，也就是你所看到的除了用来显示你所请求页面的主窗口之外的其他部分 浏览器引擎 － 用来查询及操作渲染引擎的接口 渲染引擎（浏览器内核）－ 浏览器有兼容性的根本原因，用来显示请求的内容，例如，如果请求内容为 html，它负责解析 html 及 css，并将解析后的结果显示出来 网络 － 用来完成网络调用，例如 http">
<meta property="og:type" content="article">
<meta property="og:title" content="我的博客">
<meta property="og:url" content="https://xczhello.gitee.io/uncategorized/imooc-frontend-interview/10. 渲染机制.html/index.html">
<meta property="og:site_name" content="我的博客">
<meta property="og:description" content="1. 渲染机制浏览器的组成  用户界面 － 包括地址栏、后退/前进按钮、书签目录等，也就是你所看到的除了用来显示你所请求页面的主窗口之外的其他部分 浏览器引擎 － 用来查询及操作渲染引擎的接口 渲染引擎（浏览器内核）－ 浏览器有兼容性的根本原因，用来显示请求的内容，例如，如果请求内容为 html，它负责解析 html 及 css，并将解析后的结果显示出来 网络 － 用来完成网络调用，例如 http">
<meta property="og:locale" content="zh-Hans">
<meta property="og:image" content="https://kfwimg.ikafan.com/upload/b9/19/b919dd5130b8694bea56aa44b44b32be.png">
<meta property="og:image" content="https://user-gold-cdn.xitu.io/2018/6/23/1642d16f798c0875?imageView2/0/w/1280/h/960/format/webp/ignore-error/1">
<meta property="og:updated_time" content="2019-01-05T13:47:03.000Z">
<meta name="twitter:card" content="summary">
<meta name="twitter:title" content="我的博客">
<meta name="twitter:description" content="1. 渲染机制浏览器的组成  用户界面 － 包括地址栏、后退/前进按钮、书签目录等，也就是你所看到的除了用来显示你所请求页面的主窗口之外的其他部分 浏览器引擎 － 用来查询及操作渲染引擎的接口 渲染引擎（浏览器内核）－ 浏览器有兼容性的根本原因，用来显示请求的内容，例如，如果请求内容为 html，它负责解析 html 及 css，并将解析后的结果显示出来 网络 － 用来完成网络调用，例如 http">
<meta name="twitter:image" content="https://kfwimg.ikafan.com/upload/b9/19/b919dd5130b8694bea56aa44b44b32be.png">



<script type="text/javascript" id="hexo.configurations">
  var NexT = window.NexT || {};
  var CONFIG = {
    root: '/',
    scheme: 'Pisces',
    version: '5.1.4',
    sidebar: {"position":"left","display":"post","offset":12,"b2t":true,"scrollpercent":true,"onmobile":false},
    fancybox: true,
    tabs: true,
    motion: {"enable":true,"async":false,"transition":{"post_block":"fadeIn","post_header":"slideDownIn","post_body":"slideDownIn","coll_header":"slideLeftIn","sidebar":"slideUpIn"}},
    duoshuo: {
      userId: '0',
      author: '博主'
    },
    algolia: {
      applicationID: '',
      apiKey: '',
      indexName: '',
      hits: {"per_page":10},
      labels: {"input_placeholder":"Search for Posts","hits_empty":"We didn't find any results for the search: ${query}","hits_stats":"${hits} results found in ${time} ms"}
    }
  };
</script>



  <link rel="canonical" href="https://xczhello.gitee.io/uncategorized/imooc-frontend-interview/10. 渲染机制.html/">





  <title> | 我的博客</title>
  








</head>

<body itemscope="" itemtype="http://schema.org/WebPage" lang="zh-Hans">

  
  
    
  

  <div class="container sidebar-position-left page-post-detail">
    <div class="headband"></div>

    <a href="https://github.com/Jem826" class="github-corner" aria-label="View source on GitHub"><svg width="80" height="80" viewbox="0 0 250 250" style="fill:#151513; color:#fff; position: absolute; top: 0; border: 0; right: 0;" aria-hidden="true"><path d="M0,0 L115,115 L130,115 L142,142 L250,250 L250,0 Z"/><path d="M128.3,109.0 C113.8,99.7 119.0,89.6 119.0,89.6 C122.0,82.7 120.5,78.6 120.5,78.6 C119.2,72.0 123.4,76.3 123.4,76.3 C127.3,80.9 125.5,87.3 125.5,87.3 C122.9,97.6 130.6,101.9 134.4,103.2" fill="currentColor" style="transform-origin: 130px 106px;" class="octo-arm"/><path d="M115.0,115.0 C114.9,115.1 118.7,116.5 119.8,115.4 L133.7,101.6 C136.9,99.2 139.9,98.4 142.2,98.6 C133.8,88.0 127.5,74.4 143.8,58.0 C148.5,53.4 154.0,51.2 159.7,51.0 C160.3,49.4 163.2,43.6 171.4,40.1 C171.4,40.1 176.1,42.5 178.8,56.2 C183.1,58.6 187.2,61.8 190.9,65.4 C194.5,69.0 197.7,73.2 200.1,77.6 C213.8,80.2 216.3,84.9 216.3,84.9 C212.7,93.1 206.9,96.0 205.4,96.6 C205.1,102.4 203.0,107.8 198.3,112.5 C181.9,128.9 168.3,122.5 157.7,114.1 C157.9,116.9 156.7,120.9 152.7,124.9 L141.0,136.5 C139.8,137.7 141.6,141.9 141.8,141.8 Z" fill="currentColor" class="octo-body"/></svg></a><style>.github-corner:hover .octo-arm{animation:octocat-wave 560ms ease-in-out}@keyframes octocat-wave{0%,100%{transform:rotate(0)}20%,60%{transform:rotate(-25deg)}40%,80%{transform:rotate(10deg)}}@media (max-width:500px){.github-corner:hover .octo-arm{animation:none}.github-corner .octo-arm{animation:octocat-wave 560ms ease-in-out}}</style>

    <header id="header" class="header" itemscope="" itemtype="http://schema.org/WPHeader">
      <div class="header-inner"><div class="site-brand-wrapper">
  <div class="site-meta ">
    

    <div class="custom-logo-site-title">
      <a href="/" class="brand" rel="start">
        <span class="logo-line-before"><i></i></span>
        <span class="site-title">我的博客</span>
        <span class="logo-line-after"><i></i></span>
      </a>
    </div>
      
        <p class="site-subtitle"></p>
      
  </div>

  <div class="site-nav-toggle">
    <button>
      <span class="btn-bar"></span>
      <span class="btn-bar"></span>
      <span class="btn-bar"></span>
    </button>
  </div>
</div>

<nav class="site-nav">
  

  
    <ul id="menu" class="menu">
      
        
        <li class="menu-item menu-item-home">
          <a href="/" rel="section">
            
              <i class="menu-item-icon fa fa-fw fa-home"></i> <br>
            
            首页
          </a>
        </li>
      
        
        <li class="menu-item menu-item-tags">
          <a href="/tags/" rel="section">
            
              <i class="menu-item-icon fa fa-fw fa-tags"></i> <br>
            
            标签
          </a>
        </li>
      
        
        <li class="menu-item menu-item-categories">
          <a href="/categories/" rel="section">
            
              <i class="menu-item-icon fa fa-fw fa-th"></i> <br>
            
            分类
          </a>
        </li>
      
        
        <li class="menu-item menu-item-archives">
          <a href="/archives/" rel="section">
            
              <i class="menu-item-icon fa fa-fw fa-archive"></i> <br>
            
            归档
          </a>
        </li>
      
        
        <li class="menu-item menu-item-linkfriends">
          <a href="/linkfriends/" rel="section">
            
              <i class="menu-item-icon fa fa-fw fa-link"></i> <br>
            
            友情链接
          </a>
        </li>
      
        
        <li class="menu-item menu-item-hot">
          <a href="/hot/" rel="section">
            
              <i class="menu-item-icon fa fa-fw fa-fire"></i> <br>
            
            热点关注
          </a>
        </li>
      
        
        <li class="menu-item menu-item-about">
          <a href="/about/" rel="section">
            
              <i class="menu-item-icon fa fa-fw fa-user"></i> <br>
            
            关于
          </a>
        </li>
      

      
        <li class="menu-item menu-item-search">
          
            <a href="javascript:;" class="popup-trigger">
          
            
              <i class="menu-item-icon fa fa-search fa-fw"></i> <br>
            
            搜索
          </a>
        </li>
      
    </ul>
  

  
    <div class="site-search">
      
  <div class="popup search-popup local-search-popup">
  <div class="local-search-header clearfix">
    <span class="search-icon">
      <i class="fa fa-search"></i>
    </span>
    <span class="popup-btn-close">
      <i class="fa fa-times-circle"></i>
    </span>
    <div class="local-search-input-wrapper">
      <input autocomplete="off" placeholder="搜索..." spellcheck="false" type="text" id="local-search-input">
    </div>
  </div>
  <div id="local-search-result"></div>
</div>



    </div>
  
</nav>



 </div>
    </header>

    <main id="main" class="main">
      <div class="main-inner">
        <div class="content-wrap">
          <div id="content" class="content">
            

  <div id="posts" class="posts-expand">
    

  

  
  
  

  <article class="post post-type-normal" itemscope="" itemtype="http://schema.org/Article">
  
  
  
  <div class="post-block">
    <link itemprop="mainEntityOfPage" href="https://xczhello.gitee.io/uncategorized/imooc-frontend-interview/10. 渲染机制.html/">

    <span hidden itemprop="author" itemscope="" itemtype="http://schema.org/Person">
      <meta itemprop="name" content="Yomi">
      <meta itemprop="description" content="">
      <meta itemprop="image" content="/images/head.jpg">
    </span>

    <span hidden itemprop="publisher" itemscope="" itemtype="http://schema.org/Organization">
      <meta itemprop="name" content="我的博客">
    </span>

    
      <header class="post-header">

        
        
          <h1 class="post-title" itemprop="name headline"></h1>
        

        <div class="post-meta">
          <span class="post-time">
            
              <span class="post-meta-item-icon">
                <i class="fa fa-calendar-o"></i>
              </span>
              
                <span class="post-meta-item-text">发表于</span>
              
              <time title="创建于" itemprop="dateCreated datePublished" datetime="2019-03-06T17:48:34+08:00">
                2019-03-06
              </time>
            

            

            

            
          </span>

          

          

          
            
              <span class="post-comments-count">
                <span class="post-meta-divider">|</span>
                <span class="post-meta-item-icon">
                  <i class="fa fa-comment-o"></i>
                </span>
                <a href="/uncategorized/imooc-frontend-interview/10. 渲染机制.html/#comments" itemprop="discussionUrl">
                  <span class="post-comments-count disqus-comment-count" data-disqus-identifier="uncategorized/imooc-frontend-interview/10. 渲染机制.html/" itemprop="commentCount"></span>
                </a>
              </span>
            
          

          
          

          
            <span class="post-meta-divider">|</span>
            <span class="page-pv"><i class="fa fa-file-o"></i> 阅读数
            <span class="busuanzi-value" id="busuanzi_value_page_pv"></span>
            </span>
          

          
            <div class="post-wordcount">
              
                
                <span class="post-meta-item-icon">
                  <i class="fa fa-file-word-o"></i>
                </span>
                
                  <span class="post-meta-item-text">字数统计&#58;</span>
                
                <span title="字数统计">
                  2.1k字
                </span>
              

              
                <span class="post-meta-divider">|</span>
              

              
                <span class="post-meta-item-icon">
                  <i class="fa fa-clock-o"></i>
                </span>
                
                  <span class="post-meta-item-text">阅读时长 &asymp;</span>
                
                <span title="阅读时长">
                  7分钟
                </span>
              
            </div>
          

          

        </div>
      </header>
    

    
    
    
    <div class="post-body" itemprop="articleBody">

      
      

      
        <h1 id="1-渲染机制"><a href="#1-渲染机制" class="headerlink" title="1. 渲染机制"></a>1. 渲染机制</h1><h2 id="浏览器的组成"><a href="#浏览器的组成" class="headerlink" title="浏览器的组成"></a>浏览器的组成</h2><p><img src="https://kfwimg.ikafan.com/upload/b9/19/b919dd5130b8694bea56aa44b44b32be.png" alt="浏览器的组成"></p>
<ul>
<li>用户界面 － 包括地址栏、后退/前进按钮、书签目录等，也就是你所看到的除了用来显示你所请求页面的主窗口之外的其他部分</li>
<li>浏览器引擎 － 用来查询及操作渲染引擎的接口</li>
<li>渲染引擎（浏览器内核）－ 浏览器有兼容性的根本原因，用来显示请求的内容，例如，如果请求内容为 html，它负责解析 html 及 css，并将解析后的结果显示出来</li>
<li>网络 － 用来完成网络调用，例如 http 请求，它具有平台无关的接口，可以在不同平台上工作</li>
<li>UI 后端 － 用来绘制类似组合选择框及对话框等基本组件，具有不特定于某个平台的通用接口，底层使用操作系统的用户接口</li>
<li>JS 解释器 － 用来解释执行 JS 代码</li>
<li>数据存储 － 属于持久层，浏览器需要在硬盘中保存类似 cookie 的各种数据，HTML5 定义了 Storage 技术，这是一种轻量级完整的客户端存储技术</li>
</ul>
<h2 id="1-1-DOCTYPE"><a href="#1-1-DOCTYPE" class="headerlink" title="1.1. DOCTYPE"></a>1.1. DOCTYPE</h2><ul>
<li><p>DTD（document type definition，文档类型定义）是一系列的语法规则，用来定义 XML 或(X)HTML 的文件类型。浏览器会使用它来判断文档类型，决定使用何种协议来解析，以及切换浏览器模式。<br>解读：DTD 就是告诉浏览器我是什么文档类型，那么浏览器根据这个来判断用什么引擎来解析渲染他。</p>
</li>
<li><p>DOCTYPE 是用来声明文档类型和 DTD 规范的，一个主要的用途便是文件的合法性验证。如果文件代码不合法，那么浏览器解析时便会出一些差错。<br>解读：DOCTYPE 通知浏览器当前文档的 DTD。</p>
</li>
<li><p>常见的 DOCTYPE</p>
</li>
</ul>
<ol>
<li>HTML 5<br><code>!DOCTYPE html</code></li>
<li>HTML 4.01 Strict，严格模式，该 DTD 包含所有 HTML 元素和属性，但不包括展示型的和弃用的元素（比如 font）<br><code>&lt;!DOCTYPE HTML PUBLIC &quot;-//W3C//DTD HTML 4.01//EN&quot; &quot;http://www.w3.org/TR/html4/strict.dtd&quot;&gt;</code></li>
<li>HTML 4.01 Transitional，传统模式（宽松模式），该 DTD 包含所有 HTML 元素和属性，包括展示型的和弃用的元素（比如 font）<br><code>&lt;!DOCTYPE HTML PUBLIC &quot;-//W3C//DTD HTML 4.01 Transitional//EN&quot; &quot;http://www.w3.org/TR/html4/loose.dtd&quot;&gt;</code></li>
</ol>
<h2 id="1-2-浏览器渲染过程"><a href="#1-2-浏览器渲染过程" class="headerlink" title="1.2. 浏览器渲染过程"></a>1.2. 浏览器渲染过程</h2><p><strong>url 解析</strong>：</p>
<ol>
<li><p>用户输入 URL 地址</p>
</li>
<li><p>浏览器解析 URL 解析出主机名</p>
</li>
<li><p>浏览器将主机名转换成服务器 ip 地址（浏览器先查找本地 DNS 缓存列表 没有的话 再向浏览器默认的 DNS 服务器发送查询请求 同时缓存）</p>
</li>
<li><p>浏览器将端口号从 URL 中解析出来</p>
</li>
<li><p>浏览器建立一条与目标 Web 服务器的 TCP 连接（三次握手）</p>
</li>
<li><p>浏览器向服务器发送一条 HTTP 请求报文</p>
</li>
<li><p>服务器向浏览器返回一条 HTTP 响应报文</p>
</li>
<li><p>关闭连接 浏览器解析文档</p>
</li>
<li><p>如果文档中有资源 重复 6 7 8 动作 直至资源全部加载完毕</p>
</li>
</ol>
<p><strong>html 解析</strong>：</p>
<p><img src="https://user-gold-cdn.xitu.io/2018/6/23/1642d16f798c0875?imageView2/0/w/1280/h/960/format/webp/ignore-error/1" alt="渲染机制"></p>
<ol>
<li>将 HTML 构建成一个 DOM 树（DOM = Document Object Model 文档对象模型），DOM 树的构建过程是一个深度遍历过程：当前节点的所有子节点都构建好后才会去构建当前节点的下一个兄弟节点。</li>
<li>解析 CSS 样式，生成样式结构体</li>
<li>结合 DOM 树和 CSSOM 生成 Rendering Tree（渲染树）。注意：Rendering Tree 渲染树并不等同于 DOM 树，因为一些像 Header 或 display:none 的东西就没必要放在渲染树中了。</li>
<li>下一步操作称之为布局（Layout），顾名思义就是计算出每个节点在屏幕中的位置 layout render tree。</li>
<li>再下一步就是绘制(Paint)，即遍历 render 树，并使用浏览器 UI 后端层绘制每个节点。</li>
</ol>
<h2 id="1-3-重排-回流-Reflow"><a href="#1-3-重排-回流-Reflow" class="headerlink" title="1.3. 重排(回流) Reflow"></a>1.3. 重排(回流) Reflow</h2><p><strong>定义</strong>：DOM 结构中的各个元素都有自己的盒子（模型），这些都需要浏览器根据各种样式来计算，并根据计算结果将元素放到它该出现的位置，这个过程称为 reflow。</p>
<p><strong>触发 Reflow</strong>：（一般触发 Reflow 时都会需要 Repaint）</p>
<ol>
<li>当你增加、删除、修改 DOM 节点时；</li>
<li>当你移动 DOM 的位置，或是搞个动画的时候；</li>
<li>当你修改 CSS 样式的时候；（边距、填充、边框、宽高等）</li>
<li>当你 Resize 窗口的时候（移动端没有这个问题），或是滚动的时候；</li>
<li>当你修改网页的默认字体时；在页面加载完后避免这样做，会影响性能</li>
</ol>
<p><strong>如何减少 Reflow</strong>：</p>
<ol>
<li>尽可能限制 reflow 的影响范围。需要改变元素的样式，不要通过父级元素影响子元素。最好直接加在子元素上。</li>
<li>通过设置 style 属性改变结点样式的话，每设置一次都会导致一次 reflow。所以最好通过设置 class 的方式。</li>
<li>减少不必要的 DOM 层级（DOM depth）。改变 DOM 树中的一级会导致所有层级的改变，上至根部，下至被改变节点的子节点。这导致大量时间耗费在执行 reflow 上面。</li>
<li>避免不必要的复杂的 CSS 选择器，尤其是后代选择器（descendant selectors），因为为了匹配选择器将耗费更多的 CPU。</li>
</ol>
<h2 id="1-4-重绘-Repaint"><a href="#1-4-重绘-Repaint" class="headerlink" title="1.4. 重绘 Repaint"></a>1.4. 重绘 Repaint</h2><p><strong>定义</strong>：当各种盒子的位置、大小以及其他属性，例如颜色、字体大小等都确定下来后，浏览器于是便把这些元素按照各自的特性绘制了一遍，于是页面的内容出现了，这个过程称之为 Repaint。（解读：页面要呈现的内容，统统画在屏幕上）</p>
<p><strong>触发 Repaint</strong>：</p>
<ol>
<li>DOM 改动；</li>
<li>CSS 改动；</li>
<li>如果有一些不影响布局的样式属性修改(background-color，color，background-image)只会触发重绘，不会触发回流</li>
</ol>
<p><strong>如何尽量减少 Repaint 频率</strong>：<br>将需要追加的多个 DOM 节点先存放在一个片段（createDocumentFragment()）中，最后一次性放到页面中。</p>
<h2 id="1-5-聪明的浏览器"><a href="#1-5-聪明的浏览器" class="headerlink" title="1.5. 聪明的浏览器"></a>1.5. 聪明的浏览器</h2><blockquote>
<p>但是假设每句 JS 操作都去回流重绘的话，浏览器可能就会受不了。所以很多浏览器都会优化这些操作，浏览器会维护 1 个队列，把所有会引起回流、重绘的操作放入这个队列，等队列中的操作到了一定的数量或者到了一定的时间间隔，浏览器就会 flush 队列，进行一个批处理。这样就会让多次的回流、重绘变成一次回流重绘。</p>
</blockquote>
<p>虽然有了浏览器的优化，但有时候我们写的一些代码可能会强制浏览器提前 flush 队列，这样浏览器的优化可能就起不到作用了。当你请求向浏览器请求一些 style 信息的时候，就会让浏览器 flush 队列，比如：</p>
<p>1.offsetTop, offsetLeft, offsetWidth, offsetHeight</p>
<p>2.scrollTop/Left/Width/Height</p>
<p>3.clientTop/Left/Width/Height</p>
<p>4.请求了 getComputedStyle(), 或者 IE 的 currentStyle</p>
<p>所有获取性操作，都会让浏览器提前 flush 队列（提前批处理），浏览器是为了能够让获取性操作获取的样式保证准确性</p>
<h2 id="1-6-渲染优化"><a href="#1-6-渲染优化" class="headerlink" title="1.6. 渲染优化"></a>1.6. 渲染优化</h2><p>都是在想办法减少回流和重绘的次数</p>
<ol>
<li>HTML 文档结构层次尽量少，最好不深于六层；</li>
<li>脚本尽量后放，放在前即可；</li>
<li>少量首屏样式内联放在标签内；</li>
<li>样式结构层次尽量简单；</li>
<li>在脚本中尽量减少 DOM 操作，尽量缓存访问 DOM 的样式信息，避免过度触发回流；</li>
<li>减少通过 JavaScript 代码修改元素样式，尽量使用修改 class 名方式操作样式或动画；</li>
<li>position：fixed/absolute；定位的元素的布局属性更改不会影响到其他元素，动画尽量使用在绝对定位或固定定位的元素上；</li>
<li>css3 的变换（transform 系列）只是一种视觉效果，只会触发重绘，可以减少一次回流的过程</li>
<li>隐藏在屏幕外，或在页面滚动时，尽量停止动画；</li>
<li>尽量缓存 DOM 查找，查找器尽量简洁；</li>
<li>涉及多域名的网站，可以开启域名预解析</li>
</ol>

      
    </div>
    
    
    

    <div>
      
        <div>
    
        <div style="text-align:center;color: #ccc;font-size:14px;">-------------本文结束<i class="fa fa-paw"></i>感谢您的阅读-------------</div>
    
</div>

      
    </div>

    <div>
          
            


          
    </div>

    

    

    

    <footer class="post-footer">
      

      
      
        <div class="post-widgets">
        

        

        
          
          <div id="needsharebutton-postbottom">
            <span class="btn">
              <i class="fa fa-share-alt" aria-hidden="true"></i>
            </span>
          </div>
        
        </div>
      
      

      
        <div class="post-nav">
          <div class="post-nav-next post-nav-item">
            
              <a href="/uncategorized/imooc-frontend-interview/1. 页面布局.html/" rel="next" title="">
                <i class="fa fa-chevron-left"></i> 
              </a>
            
          </div>

          <span class="post-nav-divider"></span>

          <div class="post-nav-prev post-nav-item">
            
              <a href="/uncategorized/imooc-frontend-interview/11. JS运行机制：异步和单线程.html/" rel="prev" title="">
                 <i class="fa fa-chevron-right"></i>
              </a>
            
          </div>
        </div>
      

      
      
    </footer>
  </div>
  
  
  
  </article>



    <div class="post-spread">
      
    </div>
  </div>


          </div>
          


          

  
    <div class="comments" id="comments">
      <div id="disqus_thread">
        <noscript>
          Please enable JavaScript to view the
          <a href="https://disqus.com/?ref_noscript">comments powered by Disqus.</a>
        </noscript>
      </div>
    </div>

  



        </div>
        
          
  
  <div class="sidebar-toggle">
    <div class="sidebar-toggle-line-wrap">
      <span class="sidebar-toggle-line sidebar-toggle-line-first"></span>
      <span class="sidebar-toggle-line sidebar-toggle-line-middle"></span>
      <span class="sidebar-toggle-line sidebar-toggle-line-last"></span>
    </div>
  </div>

  <aside id="sidebar" class="sidebar">
    
    <div class="sidebar-inner">

      

      
        <ul class="sidebar-nav motion-element">
          <li class="sidebar-nav-toc sidebar-nav-active" data-target="post-toc-wrap">
            文章目录
          </li>
          <li class="sidebar-nav-overview" data-target="site-overview-wrap">
            站点概览
          </li>
        </ul>
      

      <section class="site-overview-wrap sidebar-panel">
        <div class="site-overview">
          <div class="site-author motion-element" itemprop="author" itemscope="" itemtype="http://schema.org/Person">
            
              <img class="site-author-image" itemprop="image" src="/images/head.jpg" alt="Yomi">
            
              <p class="site-author-name" itemprop="name">Yomi</p>
              <p class="site-description motion-element" itemprop="description">--悟已往之不谏，知来者之可追--</p>
          </div>

          <nav class="site-state motion-element">

            
              <div class="site-state-item site-state-posts">
              
                <a href="/archives/">
              
                  <span class="site-state-item-count">28</span>
                  <span class="site-state-item-name">日志</span>
                </a>
              </div>
            

            
              
              
              <div class="site-state-item site-state-categories">
                <a href="/categories/index.html">
                  <span class="site-state-item-count">6</span>
                  <span class="site-state-item-name">分类</span>
                </a>
              </div>
            

            
              
              
              <div class="site-state-item site-state-tags">
                <a href="/tags/index.html">
                  <span class="site-state-item-count">5</span>
                  <span class="site-state-item-name">标签</span>
                </a>
              </div>
            

          </nav>

          

          
            <div class="links-of-author motion-element">
                
                  <span class="links-of-author-item">
                    <a href="https://github.com/Jem826" target="_blank" title="GitHub">
                      
                        <i class="fa fa-fw fa-github"></i>GitHub</a>
                  </span>
                
                  <span class="links-of-author-item">
                    <a href="https://www.jianshu.com" target="_blank" title="简书">
                      
                        <i class="fa fa-fw fa-heartbeat"></i>简书</a>
                  </span>
                
                  <span class="links-of-author-item">
                    <a href="https://juejin.im" target="_blank" title="掘金">
                      
                        <i class="fa fa-fw fa-gg-circle"></i>掘金</a>
                  </span>
                
                  <span class="links-of-author-item">
                    <a href="tencent://message/?uin=964325846&Site=个人QQ&Menu=yes" target="_blank" title="QQ">
                      
                        <i class="fa fa-fw fa-qq"></i>QQ</a>
                  </span>
                
            </div>
          

          
          

          
          
            <div class="links-of-blogroll motion-element links-of-blogroll-block">
              <div class="links-of-blogroll-title">
                <i class="fa  fa-fw fa-link"></i>
                推荐阅读
              </div>
              <ul class="links-of-blogroll-list">
                
                  <li class="links-of-blogroll-item">
                    <a href="http://www.uisdc.com/" title="优设" target="_blank">优设</a>
                  </li>
                
                  <li class="links-of-blogroll-item">
                    <a href="http://www.zhangxinxu.com/" title="张鑫旭" target="_blank">张鑫旭</a>
                  </li>
                
                  <li class="links-of-blogroll-item">
                    <a href="http://www.alloyteam.com/nav/" title="Web前端导航" target="_blank">Web前端导航</a>
                  </li>
                
                  <li class="links-of-blogroll-item">
                    <a href="http://www.36zhen.com/t?id=3448" title="前端书籍资料" target="_blank">前端书籍资料</a>
                  </li>
                
                  <li class="links-of-blogroll-item">
                    <a href="http://hao.shejidaren.com" title="设计导航" target="_blank">设计导航</a>
                  </li>
                
                  <li class="links-of-blogroll-item">
                    <a href="http://www.vipsheji.cn" title="设计网站大全" target="_blank">设计网站大全</a>
                  </li>
                
                  <li class="links-of-blogroll-item">
                    <a href="http://ife.baidu.com/" title="百度前端技术学院" target="_blank">百度前端技术学院</a>
                  </li>
                
                  <li class="links-of-blogroll-item">
                    <a href="http://wf.uisdc.com/cn/" title="google前端开发基础" target="_blank">google前端开发基础</a>
                  </li>
                
              </ul>
            </div>
          

          

        </div>
      </section>

      
      <!--noindex-->
        <section class="post-toc-wrap motion-element sidebar-panel sidebar-panel-active">
          <div class="post-toc">

            
              
            

            
              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-1"><a class="nav-link" href="#1-渲染机制"><span class="nav-text">1. 渲染机制</span></a><ol class="nav-child"><li class="nav-item nav-level-2"><a class="nav-link" href="#浏览器的组成"><span class="nav-text">浏览器的组成</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-1-DOCTYPE"><span class="nav-text">1.1. DOCTYPE</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-2-浏览器渲染过程"><span class="nav-text">1.2. 浏览器渲染过程</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-3-重排-回流-Reflow"><span class="nav-text">1.3. 重排(回流) Reflow</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-4-重绘-Repaint"><span class="nav-text">1.4. 重绘 Repaint</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-5-聪明的浏览器"><span class="nav-text">1.5. 聪明的浏览器</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-6-渲染优化"><span class="nav-text">1.6. 渲染优化</span></a></li></ol></li></ol></div>
            

          </div>
        </section>
      <!--/noindex-->
      

      
        <div class="back-to-top">
          <i class="fa fa-arrow-up"></i>
          
            <span id="scrollpercent"><span>0</span>%</span>
          
        </div>
      

    </div>
  </aside>


        
      </div>
    </main>

    <footer id="footer" class="footer">
      <div class="footer-inner">
        <div class="copyright">&copy; <span itemprop="copyrightYear">2019</span>
  <span class="with-love">
    <i class="fa fa-user"></i>
  </span>
  <span class="author" itemprop="copyrightHolder">Yomi</span>

  
</div>

<!--

  <div class="powered-by">由 <a class="theme-link" target="_blank" href="https://hexo.io">Hexo</a> 强力驱动</div>




  <span class="post-meta-divider">|</span>



  <div class="theme-info">主题 &mdash; <a class="theme-link" target="_blank" href="https://github.com/iissnan/hexo-theme-next">NexT.Pisces</a> v5.1.4</div>



-->

        
<div class="busuanzi-count">
  <script async src="https://busuanzi.ibruce.info/busuanzi/2.3/busuanzi.pure.mini.js"></script>

  
    <span class="site-uv">
      <i class="fa fa-user"></i> 访问人数
      <span class="busuanzi-value" id="busuanzi_value_site_uv"></span>
      
    </span>
  

  
    <span class="site-pv">
      <i class="fa fa-eye"></i> 总访问量
      <span class="busuanzi-value" id="busuanzi_value_site_pv"></span>
      
    </span>
  
</div>








        
      </div>
    </footer>

    

    

  </div>

  

<script type="text/javascript">
  if (Object.prototype.toString.call(window.Promise) !== '[object Function]') {
    window.Promise = null;
  }
</script>









  












  
  
    <script type="text/javascript" src="/lib/jquery/index.js?v=2.1.3"></script>
  

  
  
    <script type="text/javascript" src="/lib/fastclick/lib/fastclick.min.js?v=1.0.6"></script>
  

  
  
    <script type="text/javascript" src="/lib/jquery_lazyload/jquery.lazyload.js?v=1.9.7"></script>
  

  
  
    <script type="text/javascript" src="/lib/velocity/velocity.min.js?v=1.2.1"></script>
  

  
  
    <script type="text/javascript" src="/lib/velocity/velocity.ui.min.js?v=1.2.1"></script>
  

  
  
    <script type="text/javascript" src="/lib/fancybox/source/jquery.fancybox.pack.js?v=2.1.5"></script>
  


  


  <script type="text/javascript" src="/js/src/utils.js?v=5.1.4"></script>

  <script type="text/javascript" src="/js/src/motion.js?v=5.1.4"></script>



  
  


  <script type="text/javascript" src="/js/src/affix.js?v=5.1.4"></script>

  <script type="text/javascript" src="/js/src/schemes/pisces.js?v=5.1.4"></script>



  
  <script type="text/javascript" src="/js/src/scrollspy.js?v=5.1.4"></script>
<script type="text/javascript" src="/js/src/post-details.js?v=5.1.4"></script>



  


  <script type="text/javascript" src="/js/src/bootstrap.js?v=5.1.4"></script>



  


  

    
      <script id="dsq-count-scr" src="https://true.disqus.com/count.js" async></script>
    

    
      <script type="text/javascript">
        var disqus_config = function () {
          this.page.url = 'https://xczhello.gitee.io/uncategorized/imooc-frontend-interview/10. 渲染机制.html/';
          this.page.identifier = 'uncategorized/imooc-frontend-interview/10. 渲染机制.html/';
          this.page.title = '';
        };
        var d = document, s = d.createElement('script');
        s.src = 'https://true.disqus.com/embed.js';
        s.setAttribute('data-timestamp', '' + +new Date());
        (d.head || d.body).appendChild(s);
      </script>
    

  




	





  














  

  <script type="text/javascript">
    // Popup Window;
    var isfetched = false;
    var isXml = true;
    // Search DB path;
    var search_path = "search.xml";
    if (search_path.length === 0) {
      search_path = "search.xml";
    } else if (/json$/i.test(search_path)) {
      isXml = false;
    }
    var path = "/" + search_path;
    // monitor main search box;

    var onPopupClose = function (e) {
      $('.popup').hide();
      $('#local-search-input').val('');
      $('.search-result-list').remove();
      $('#no-result').remove();
      $(".local-search-pop-overlay").remove();
      $('body').css('overflow', '');
    }

    function proceedsearch() {
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay"></div>')
        .css('overflow', 'hidden');
      $('.search-popup-overlay').click(onPopupClose);
      $('.popup').toggle();
      var $localSearchInput = $('#local-search-input');
      $localSearchInput.attr("autocapitalize", "none");
      $localSearchInput.attr("autocorrect", "off");
      $localSearchInput.focus();
    }

    // search function;
    var searchFunc = function(path, search_id, content_id) {
      'use strict';

      // start loading animation
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay">' +
          '<div id="search-loading-icon">' +
          '<i class="fa fa-spinner fa-pulse fa-5x fa-fw"></i>' +
          '</div>' +
          '</div>')
        .css('overflow', 'hidden');
      $("#search-loading-icon").css('margin', '20% auto 0 auto').css('text-align', 'center');

      $.ajax({
        url: path,
        dataType: isXml ? "xml" : "json",
        async: true,
        success: function(res) {
          // get the contents from search data
          isfetched = true;
          $('.popup').detach().appendTo('.header-inner');
          var datas = isXml ? $("entry", res).map(function() {
            return {
              title: $("title", this).text(),
              content: $("content",this).text(),
              url: $("url" , this).text()
            };
          }).get() : res;
          var input = document.getElementById(search_id);
          var resultContent = document.getElementById(content_id);
          var inputEventFunction = function() {
            var searchText = input.value.trim().toLowerCase();
            var keywords = searchText.split(/[\s\-]+/);
            if (keywords.length > 1) {
              keywords.push(searchText);
            }
            var resultItems = [];
            if (searchText.length > 0) {
              // perform local searching
              datas.forEach(function(data) {
                var isMatch = false;
                var hitCount = 0;
                var searchTextCount = 0;
                var title = data.title.trim();
                var titleInLowerCase = title.toLowerCase();
                var content = data.content.trim().replace(/<[^>]+>/g,"");
                var contentInLowerCase = content.toLowerCase();
                var articleUrl = decodeURIComponent(data.url);
                var indexOfTitle = [];
                var indexOfContent = [];
                // only match articles with not empty titles
                if(title != '') {
                  keywords.forEach(function(keyword) {
                    function getIndexByWord(word, text, caseSensitive) {
                      var wordLen = word.length;
                      if (wordLen === 0) {
                        return [];
                      }
                      var startPosition = 0, position = [], index = [];
                      if (!caseSensitive) {
                        text = text.toLowerCase();
                        word = word.toLowerCase();
                      }
                      while ((position = text.indexOf(word, startPosition)) > -1) {
                        index.push({position: position, word: word});
                        startPosition = position + wordLen;
                      }
                      return index;
                    }

                    indexOfTitle = indexOfTitle.concat(getIndexByWord(keyword, titleInLowerCase, false));
                    indexOfContent = indexOfContent.concat(getIndexByWord(keyword, contentInLowerCase, false));
                  });
                  if (indexOfTitle.length > 0 || indexOfContent.length > 0) {
                    isMatch = true;
                    hitCount = indexOfTitle.length + indexOfContent.length;
                  }
                }

                // show search results

                if (isMatch) {
                  // sort index by position of keyword

                  [indexOfTitle, indexOfContent].forEach(function (index) {
                    index.sort(function (itemLeft, itemRight) {
                      if (itemRight.position !== itemLeft.position) {
                        return itemRight.position - itemLeft.position;
                      } else {
                        return itemLeft.word.length - itemRight.word.length;
                      }
                    });
                  });

                  // merge hits into slices

                  function mergeIntoSlice(text, start, end, index) {
                    var item = index[index.length - 1];
                    var position = item.position;
                    var word = item.word;
                    var hits = [];
                    var searchTextCountInSlice = 0;
                    while (position + word.length <= end && index.length != 0) {
                      if (word === searchText) {
                        searchTextCountInSlice++;
                      }
                      hits.push({position: position, length: word.length});
                      var wordEnd = position + word.length;

                      // move to next position of hit

                      index.pop();
                      while (index.length != 0) {
                        item = index[index.length - 1];
                        position = item.position;
                        word = item.word;
                        if (wordEnd > position) {
                          index.pop();
                        } else {
                          break;
                        }
                      }
                    }
                    searchTextCount += searchTextCountInSlice;
                    return {
                      hits: hits,
                      start: start,
                      end: end,
                      searchTextCount: searchTextCountInSlice
                    };
                  }

                  var slicesOfTitle = [];
                  if (indexOfTitle.length != 0) {
                    slicesOfTitle.push(mergeIntoSlice(title, 0, title.length, indexOfTitle));
                  }

                  var slicesOfContent = [];
                  while (indexOfContent.length != 0) {
                    var item = indexOfContent[indexOfContent.length - 1];
                    var position = item.position;
                    var word = item.word;
                    // cut out 100 characters
                    var start = position - 20;
                    var end = position + 80;
                    if(start < 0){
                      start = 0;
                    }
                    if (end < position + word.length) {
                      end = position + word.length;
                    }
                    if(end > content.length){
                      end = content.length;
                    }
                    slicesOfContent.push(mergeIntoSlice(content, start, end, indexOfContent));
                  }

                  // sort slices in content by search text's count and hits' count

                  slicesOfContent.sort(function (sliceLeft, sliceRight) {
                    if (sliceLeft.searchTextCount !== sliceRight.searchTextCount) {
                      return sliceRight.searchTextCount - sliceLeft.searchTextCount;
                    } else if (sliceLeft.hits.length !== sliceRight.hits.length) {
                      return sliceRight.hits.length - sliceLeft.hits.length;
                    } else {
                      return sliceLeft.start - sliceRight.start;
                    }
                  });

                  // select top N slices in content

                  var upperBound = parseInt('1');
                  if (upperBound >= 0) {
                    slicesOfContent = slicesOfContent.slice(0, upperBound);
                  }

                  // highlight title and content

                  function highlightKeyword(text, slice) {
                    var result = '';
                    var prevEnd = slice.start;
                    slice.hits.forEach(function (hit) {
                      result += text.substring(prevEnd, hit.position);
                      var end = hit.position + hit.length;
                      result += '<b class="search-keyword">' + text.substring(hit.position, end) + '</b>';
                      prevEnd = end;
                    });
                    result += text.substring(prevEnd, slice.end);
                    return result;
                  }

                  var resultItem = '';

                  if (slicesOfTitle.length != 0) {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + highlightKeyword(title, slicesOfTitle[0]) + "</a>";
                  } else {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + title + "</a>";
                  }

                  slicesOfContent.forEach(function (slice) {
                    resultItem += "<a href='" + articleUrl + "'>" +
                      "<p class=\"search-result\">" + highlightKeyword(content, slice) +
                      "...</p>" + "</a>";
                  });

                  resultItem += "</li>";
                  resultItems.push({
                    item: resultItem,
                    searchTextCount: searchTextCount,
                    hitCount: hitCount,
                    id: resultItems.length
                  });
                }
              })
            };
            if (keywords.length === 1 && keywords[0] === "") {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-search fa-5x" /></div>'
            } else if (resultItems.length === 0) {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-frown-o fa-5x" /></div>'
            } else {
              resultItems.sort(function (resultLeft, resultRight) {
                if (resultLeft.searchTextCount !== resultRight.searchTextCount) {
                  return resultRight.searchTextCount - resultLeft.searchTextCount;
                } else if (resultLeft.hitCount !== resultRight.hitCount) {
                  return resultRight.hitCount - resultLeft.hitCount;
                } else {
                  return resultRight.id - resultLeft.id;
                }
              });
              var searchResultList = '<ul class=\"search-result-list\">';
              resultItems.forEach(function (result) {
                searchResultList += result.item;
              })
              searchResultList += "</ul>";
              resultContent.innerHTML = searchResultList;
            }
          }

          if ('auto' === 'auto') {
            input.addEventListener('input', inputEventFunction);
          } else {
            $('.search-icon').click(inputEventFunction);
            input.addEventListener('keypress', function (event) {
              if (event.keyCode === 13) {
                inputEventFunction();
              }
            });
          }

          // remove loading animation
          $(".local-search-pop-overlay").remove();
          $('body').css('overflow', '');

          proceedsearch();
        }
      });
    }

    // handle and trigger popup window;
    $('.popup-trigger').click(function(e) {
      e.stopPropagation();
      if (isfetched === false) {
        searchFunc(path, 'local-search-input', 'local-search-result');
      } else {
        proceedsearch();
      };
    });

    $('.popup-btn-close').click(onPopupClose);
    $('.popup').click(function(e){
      e.stopPropagation();
    });
    $(document).on('keyup', function (event) {
      var shouldDismissSearchPopup = event.which === 27 &&
        $('.search-popup').is(':visible');
      if (shouldDismissSearchPopup) {
        onPopupClose();
      }
    });
  </script>





  

  

  

  
  
  
  <link rel="stylesheet" href="/lib/needsharebutton/needsharebutton.css">

  
  
  <script src="/lib/needsharebutton/needsharebutton.js"></script>

  <script>
    
      pbOptions = {};
      
          pbOptions.iconStyle = "box";
      
          pbOptions.boxForm = "horizontal";
      
          pbOptions.position = "bottomCenter";
      
          pbOptions.networks = "Weibo,Wechat,Douban,QQZone,Twitter,Facebook,GooglePlus,Linkedin,Evernote";
      
      new needShareButton('#needsharebutton-postbottom', pbOptions);
    
    
  </script>

  

  

  

  

  <!-- 页面点击小红心 -->
  <script type="text/javascript" src="/js/src/click.js"></script>
  <!-- 背景动画 -->
  <script type="text/javascript" src="/js/src/particle.js"></script>
<script src="/live2dw/lib/L2Dwidget.min.js?0c58a1486de42ac6cc1c59c7d98ae887"></script><script>L2Dwidget.init({"pluginRootPath":"live2dw/","pluginJsPath":"lib/","pluginModelPath":"assets/","tagMode":false,"debug":false,"model":{"jsonPath":"/live2dw/assets/tororo.model.json"},"display":{"position":"right","width":120,"height":240},"mobile":{"show":true},"log":false});</script></body>
</html>
